Vulkan: Attempt to fix the tsan complain regarding volk. In certain cases, I am seeing volk function pointer been set while it is still being accessed. See https://chromium-swarm.appspot.com/task?id=60b37aa811f8b410. My guess it is related to some tests configs that end up with RendererVk gets initialized while other instance is still running. This is attempt to fix the tsan complain by only load volk function pointer if needed. Bug: angleproject:8055 Change-Id: I5dcc62876bf93a250080de1f8e1b97d2b7570ab8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4300072 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Charlie Lao <cclao@google.com>